An alternating current can induce voltage because it has

1. Analyze the question

The question asks for the fundamental reason why an alternating current (AC) is capable of inducing a voltage in a nearby conductor or coil.

2. Explain the concept

This phenomenon is based on Faraday’s Law of Electromagnetic Induction. Faraday’s Law states that an electromotive force (EMF) or voltage is induced in a circuit whenever there is a change in the magnetic flux linked with that circuit.

• Direct Current (DC): Flows in one constant direction. If the current is steady, it produces a constant magnetic field. A constant magnetic field does not change, so it cannot induce a voltage in a stationary secondary coil.
• Alternating Current (AC): Constantly changes its magnitude and reverses its direction periodically. Because the current is always changing, the magnetic field it produces around the conductor is also constantly changing (varying). This changing magnetic field cuts across nearby conductors, satisfying the condition for electromagnetic induction.

3. Show the solution

• Step 1: Identify the requirement for induction. According to Faraday's Law, $e = -N \frac{d\Phi}{dt}$, where $e$ is the induced voltage and $\frac{d\Phi}{dt}$ is the rate of change of magnetic flux.
• Step 2: Relate the current to the magnetic field. A current-carrying conductor creates a magnetic field ($B$). For AC, $I = I_0 \sin(\omega t)$, which means the magnetic field $B$ is also a function of time.
• Step 3: Conclude the effect. Since the current varies with time, the magnetic field varies with time. This variation ($\frac{dB}{dt}$) is what induces the voltage in a transformer or a secondary coil.

4. Identify the answer

Comparing this to the options:

• A is incorrect because "ripple" refers to the fluctuations in rectified DC, not the fundamental cause of induction.
• B is correct because the varying magnetic field is the direct physical mechanism that satisfies Faraday's Law.
• C is incorrect because the strength of the field is not the primary factor; the change in the field is.
• D is incorrect because peak value relates to amplitude, not the induction process itself.

5. Tip

Exam Strategy: Whenever you see a question about "induction" (transformers, generators, or induction coils), look for keywords like "changing," "varying," "cutting," or "rate of change." If the magnetic field isn't changing, no induction occurs!